Light sensitive circuit



p 1942- w. J. ALBRSHEIM 2,295,536

LIGHT SENSITIVE CIRCUIT 2 Sheets-Sheet 1 Filed Oct. 8, 1941 IN [/5 N TORW. J. ALBERS/vf/M TM. Au br. A7 TORNE V p 5, 1942- w. J. ALBERSHElM2,295,536

LIGHT SENSITIVE CIRCUIT Filed Oct. 8, 1941 2 Sheets-Sheet 2 All! .//V V:N TOR W. J. .A L BERSHE/M A 7' TORNE V Patented Sept. 15, 1 942 UNITEDSTATES PATENT} OFFICE- LIGHT SENSITIVE CIRCUIT Walter J. Albersheim,Great Neck, N. Y., assignor to Western Electric Company, Incorporated, acorporation of New York Application October 8, 1941, Serial No. 414,083

This invention relates to improvements in 16 Claims.

two anodes and a common cathode in the same glass envelope.

In a photocell of this type it is desired that no light intended for oneportion only of the cathode shall reach the remaining cathode portionand further that no electrons or other charged par-- ticles belonging tothe current conduction from one anode to its corresponding cathodeportion shall stray over to take part in the current conduction from theother anode to the other portion of the cathode. In practice it is foundthat neither of the foregoing requirements is fully met. In the WesternElectric 9A cell, for example, optical and electronic crosstalk isgreatly reduced by the cell construction shown in U. S.

Patent 2,198,650 above referred to, but enough crosstalk remains tobring it about that if the two currents corresponding to the separateanode-cathode paths are equal, a small fraction, say 5 per cent, of eachsuch current is due to crosstalk from the other conducting path. Opticalcrosstalk arises from direct incidence of light on the cathode portionfor which it is not intended and also from scattered reflections oflight within the cell. Electronic crosstalk arises in part fromelectrons which escape their approtion in volume of a few tenths of adecibel. Again where the two film tracks are sound records correspondingto the same sound source but with different microphone positions such asstereophonic records, the only effect of crosstalk will be i a slightloss in effective localization, whereas the quality is not impairedbecause, as in the pushpull case, the two records are of the sameacoustic character.

However, it has been proposed to use such a f photocell for thereproduction of two parallel film tracks of which one is a sound trackand the other is a record related to the sound record but distincttherefrom, such as a volume control track for regulation of the gain ofthe reproducing amplifier. In this case, since the control track isusually a record of a high frequency modulated by the envelope of thesound signal, crosstalk from it into the sound record part of the cellintroduces noises fatal to good sound reproduction. Crosstalk from thesound record into the control'track reproduction has the effect ofconfusing the gain controls.

The object of this invention is the substan- Q tially completecompensation of undesired currents such as the crosstalk above describedin the separate outputs of photoelectric cells having two or more anodesanda cathode commonthereto, for the types of sound film reproductionabove enumerated or for any other application in which it may beproposed to use such photocells. This object is attained by providing amethod and circuits adapted to eliminate the undesired voltage priateanode to reach the other anode, in part" from scattering of conductingparticles released from the glass envelope by electronic bombardment,but chiefly from secondary electronsdue to collisions betweenphotoelectrons and gas molecules.

There are various purposes for which such photocells with plural anodesand common cathode may be used in the reproduction of sound films. Thecells in practical use are provided with two anodes and a common cathodeand are customarily called push-pull cells because they have beenprincipally used for the reproduction of two sound records side by sideon the same film, both being records of the same sound source, buteverywhere difiering in phase by 180 degrees. In such a case theexistence of crosstalk of the low value characteristic of the 9A cellresults in no perceptible damage to the reproduced sound, the efiectbeing only a reduccomponents from the grids of amplifying tubesappropriately connected each to one of the photocell outputs whileleaving the desired voltage components eflective onsaid grids at nearlythe same values as if crosstalk were wholly absent.

Fig. 4 is an extension of the circuit of Fig. 3, enabling this inventionto be applied to a photocell having three anodes and a common cathode.

In all figures corresponding parts are identified by the same referencenumerals.

Referring to Fig. 1 two light beams L1 and L2, which may be transmittedfrom a single light source through two separate film tracks, areincident on the photosensitive surface of cathode 4 of photocell l whichcell is provided with two anodes to which cathode 4 is common. Theselight beams are intended each to illuminate one portion only of surface4, as L1 for portion 2, and L2 for portion 3. however, some of L1reaches portion 3 and some of L2 reaches portion 4. A voltage of, say 90volts from battery 5 is applied through polarizing resistor 1 to thephotocell anode 6 facing cathode portion 2, and through polarizingresistor 9 to anode 8 facing cathode portion 3. Under the influence ofthe incident beams L1 and'L2, photoelectric currents flow in the twoportionsof the cell incompletely separated by cusp I of cathode 4 and aspreviously explained, each of these. curcents contains a small componentproperly belonging to the other.

The grids H and I2 of amplifying tubes VI and V2 respectively areconnected to cell anodes 6 and 8, grid H to, anode 6 through capacitor13, grid l2 to anode 8 through capacitor I4. Voltage to plates. l and N5of the amplifying tubes may be supplied as in Fig. 1 from the positiveterminal of.battery 5 through the primaries. of; transformers TI and T2,respectively, or maybe supplied from a separate voltage source.Cathodes' and [8 of tubes VI and V2, respectively, areconnected toground and the negative terminal ofbattery 5 (orother voltage supply)through the conventional biasing resistors and by-pass condensers asindicated by 19 and 20 in Fig. 1.. The filament heating circuits are notshown.. The tube outputs are taken from the secondary terminals oftransformers TI and T2. In the conventional circuit mesh connecting gridand cathode of an amplifying tube to the output terminals of thephotocell, the photocell cathode is connected to ground, to the negativebattery terminal and to the end of the tube grid leak remote from thegrid. According to this invention, however, a potentiometer 23 isinserted between cathode 4 and the ground line G. Grid leaks 2| and 22are connectedas shown-in Fig. 1 to separately adjustable taps 24 and 25,respectively, on potentiometer 23 instead of directly to ground line G.The insertion of potentiometer 23 between cathode 4 and ground line Gand the separate connection of grid leaks 2| and 22 to adjustable tapson potentiometer 23 constitute the circuit improvement of this inventionas applied in solving the problem of amplifying separately and withoutmutual contamination modulated photocell currents corresponding tomodulations of light flux in the incident beams L1 and L2. Beams L1 andL2 may be modulated, for example, by a sound record and by a volumecontrol record, respectively, on a photographic film. Such a film andthemechanism for feeding it past. an exciting light source, together withthe optical system for illuminating the film and effecting theseparation of beams L1 and L2 are not shown, inasmuch as they form nopart of this invention;

How the method of the invention attains the desired object will'beapparent from the following discussion with references therein to Fig.2. If, for example, photocell l is a Western Electric As indicated inFig. 1,

rent in the potentiometer inserted between the cell cathode and ground.In each anode-cathode path the ratio of desired to undesired current is95 to 5, but the currents to be separated appear in thecathode-to-ground potentiometer 24, each equal to the total current ineither cell path.

The same distributions are characteristic of the alternating currentsprovoked in the photocell by modulation of the light beams L1 and L2provided these beams are both modulated to the same extent. Therefore ifamplifying tubes, which are suitably Western Electric 2623 tubes, areconnected one each to one of the photocell paths in the conventionalmanner the grid voltage of each tube will contain desired and undesiredcomponents also in the ratio 95 to 5. The current in the returnconductorfrom cathode 4 to ground line G will consist of two approximately equalcurrents each corresponding to modulation of one of the light beams L1and L2. The voltage drop across potentiometer 23 inserted according tothis invention, therefore also consists of two approximately equalcomponents. It will be understood that potentiometer 23 may comprise twoparallel resistorsprovided one with tap 24, the other with tap 25.

Fig. 2, a circuit symbolically equivalent to that of Fig. 1, exhibitsinstantaneous relationships of the alternating currents therein. Forsimplicity, battery 5, capacitors l3 and 14 are replaced byshort-circuit connections since Fig. 2 concerns only alternatingcurrents. It is an object of this invention to impress upon grid 11- avoltage proportional only to L1 and upon grid l2 a voltage proportionalonly to L2.

In Fig. 2 the right-hand anode-cathode path of photo-cell l isrepresented by current source i1 connected to the external circuitsthrough a very high internal resistance 28 of the order of 100 megohms.Similarly, 22 represents the lefthand path of photocell 4 as a currentsource in series with internal resistance 29 also of the order of 100megohms.

'In the following analysis, let in and i 2 be the photoelectricefiiciency of cathode portions 2 and 3respectively, S1 and S2 be thefractions of light beams L1 and L2 respectively, which fall undesirablyS1 on cathode portion 3 and S2 on oathode portion 2. Then the cathodeemission will be Most of the current i1 will flow to anode 6, most of 2to anode 8' in cell 1, but the portion of each current will be divertedby electrical crosstalk represented in Fig. 2 by resistances 26 and 21.In the case of 5 per cent crosstalk resistances 26 and 21 are each ofthe order of 2000 megohms or twenty times resistance 28 or 29. In thediagram of Fig. 2 2'21 and iaZ are the currents actually reaching anodes6 and 8, respec- 9A cell and if light beams L1 and L2 are of equaltively. Equations 1 and 2 apply to the steady currents corresponding tounmodulated light beams L1 and L2. These equations apply equally to thealternating currents which correspond to modulation of light, it havingbeen assumed that L1 and L2 are modulated to the same extent. In Whatfollows, i1 and 2'2 will be taken to be alternating currents, L1 and L2modulated light amplitudes.

In the equations below, each resistance value is ildenigfized by theappropriate numerals in Figs.

Considering the mesh of Fig. 2, one finds the total current to anode 6:

R27 R28 RT1+'R29 BEER Multiplying Equations 1 and 3, we find Theapproximation involves neglecting resistances R1 and R9, which are ofthe order of 1 megohm, and so negligible in comparison with the internalcell resistances R28 and R29 and with the still greater crosstalkresistances R26 and R21.

Simplifying Equation 5, one may write al l In evaluating the voltage e1between grid II and ground, it is convenient to consider as positivecurrent flowing toward grid H in grid leak 2|, as negative currentflowing toward ground in potentiometer 23. The voltage on grid llcontributed by 2'21 is ea1=ia1Z1 where Z1 is the actual resistancebetween grid ll and ground G. Approximately, R1( 21+R24) R7121R7+R21+R24 R7+R21 since R24, the resistance included between grid andtap 24 on potentiometer 23, is much smaller than R7 and R21.

With this approximation,

R7R21 R7+R21 (9) The voltage contributed by current is in potentiometer23 is to the same approximation as (9). Combining (9) and (10) one findse1=e21+e3, or

R l= T ai zi s z4) It is desired that e1 be independent of L2. By

substituting in (11) the components of 7:21 and is which depend on L2,Equations 6 and 8, one finds that L2 disappears from (11) if that is, if

Q2R21=R24 (13) From (1 1) and (13) one finds ei=,fj isozia) (14)Substituting in (14) th L1 components of (6) and (8), one findsAdjusting in the same fashion tap 25 on pctentiometer 23, one obtains bysimilar reasoning for the voltage on grid I2 The described adjustmentsof taps 24 and 25 permit the electrical reproduction of light signals L1and L2 independently of each other. In practice, these adjustments arepreferably made as follows, since Q1, Q2 are usually unknown:

Light L1 is obscured, and tap 24 is adjusted by trial to minimize thecurrent (now solely due to light L2) in the output circuit of VI. Afterthis adjustment of tap 24, L1 is restored, L2 is obscured and tap 25 isadjusted to minimize the current due to L1 in the output circuit of V2.

Having made these adjustments of taps 24 and 25, one obtains voltages e1and e2 on grids H and I2, respectively, independent of each other. Eachof the voltages e1 and 62 is less than it would be were there nocrosstalk, in the ratio 1-Q1Q2 to 1. Thus, the object of this inventionis attained.

Those skilled in the art will recognize that grids II and [2 may becoupled conductively to anodes 6 and 8, respectively, and capacitativelyto grid leaks 2| and 22, respectively, provided appropriate changes aremade in the supply circuits to tubes VI and V2, without departing fromthe spirit of this invention.

In the circuit of Fig. 1 and its symbolic equivalent Fig. 2, the twosignal currents which are to be separated are superimposed in greatlydiffering ratios in difierent circuit elements. Under the conditionsassumed to illustrate the application of this invention, the twocurrents appear with approximately equal values in potentiometer 23, butin either anode-cathode path of cell I these currents are in the ratioof about to 5. According to the method of this invention, the separationis effected by introducing in series with the normal grid-to-groundvoltage of each amplifying tube a voltage of opposing phase and ofsuitable magnitude derived from potentiometer 23, to effect cancellationof the undesired component at the expense of cancelling at the same timea small fraction of the desired component.

As a numerical example, assume:

L1=L2=0.01 lumen (direct current component) 1 I 2=100 microamperes perlumen S1=S2=0.02

R21=R22=0.5 megohm With the above constants, Equation 5 givesia1=ia2=0.936 L1+0.064 L2 (direct current component) In this case,Equation 13 gives R24=R25=32,000 ohms Equation 15 gives 61:0.29 M1 Volte2=0.29 M2 volt ode and the ground line.

where M1 and M2 .arethe modulationiactors of light fluxes L1 and L2,respectively; c1 and 62 are the maximum instantaneous values of thealternating voltages on grids II and I2, respectively. Were there nocrossstalk, enand eg -would:

each be greater than the values given in the ratio 1/1(2 .0.064) or1.15.

The method of this invention isapplicable not solely where the undesiredvoltages'are compensatedby voltagesof thesame kind and of Terminal 24 ofgrid leak 2 is connected to a tap ion-potentiometer 35; :terminal 25015grid leak 22 is connected correspondingly :to potentiom- .eter;32. Othercircuit-elements are identically as in :Fig. 1. Sincetheg-rid and :platevoltages of each Itubeare opposite -in phase, the desired and'undes'rredvoltage components on the grids have-counterparts-of opposite phase .inthe plate circuits amplified inthe same ratio.

'Inthe-description of Figs. 1 'and'2, it was stated that the resistanceof potentiometer 23 should Tbeapproxima'tely 2Q times R21. In Fig. 3 theresistance :o'f potentiometers 32 and 35 'may be suit-ably chosen asfollows:

The 'grid voltages, proportional approximately to R21 and R22, areamplified to be proportional .to these resistances multiplied :by

where is the amplification factor of the .tube *and Rexa, Rm. are theload resistance .(through theoutput transformer) and the internalresistance of the tube, respectively. Therefore, to obtain acompensating voltage proportional to 2Q R21, the resistance ofpotentiometer 32 should bear to thetotal resistance Rs1+Rs2 the ratio Inthese formulas R32 and R35 may be neglected, since they are smallcompared to R31 and R34, respectively.

Resistors R31 and Rai arechosen of much (say 100 times) higherresistance than the load resistances. As an example, the load resistanceof each tube may be 110,000 ohms and the internal tube resistance"20,000ohms. In this case R31 and R34 are each suitably 1 megohm. If now ,u:16and 2Q=0.10,

Accordingly, 25,000 ohm potentiometersiare suitable for R32 and R35.

Condensers and must ibe .o'f negligible Such an application 5 ill] 46and. 48.

impedance compared to Bar and R34-at the lowest frequency to betransmitted; :for the ;present purpose they are suitably chosen 0.1microfarad. each.

If the load impedance of either tube is not a pureresistance,compensation .for its reactive component must be provided inthe shunt 30--3|32 (or -33-34-35-). This may be done by shunting R31(orR3 with an impedance of the same character as the load impedance,bearing to the latter the same ratio that R31 (or R34) bears to theinternal tube resistance; other expedients for this purpose will occurto those employing the invention.

The impedance through which voltage is supplied from battery '5 to thetubeplates l5 and I6 and to the cell anodes 6 and 8 are readily :chosenby-one skilled in the 'art'who has before him the characteristics of theamplifying tubes and of the photocell he has elected to use. The gridleaks 2| and '22 are likewise readily chosen; in the illustration(Fig. 1) of the present invention, using the Western Electric 9Aphotocell and 2623 tubes, suitable values are as follows:

and the biasing resistors with by-pass condensers l9 and 20,suitably-comprise each a resistance of 1000 ohms in shunt with 'l if. Itmay be desired to .take the amplified photocell voltages from plates l5and I6 of tubes VI and V2 to subsequent amplifiers; in this casetransformers TI and T2 may suitably be replaced byresistances of about10,000 ohms. For the circuit of Fig. 3 the same resistors and capacitorsare appropriate as for the circuit of'Fig. 1. It has already been statedhow those employing this invention may choose the resistances ofpotentiometer 23 of Fig. 1 and of potentiometers32 and 350i Fig. 3.

The alternative circuit of Fig. 3 permits the application of theinvention to the case of photocells having three or more anodes and acommon cathode. How this is accomplished is'shown in Fig. 4, where cellI has a third anode 36 and an additional cusp l0. Voltage to anode 36 issupplied from battery '5'through resistor 31. Amplifying tube V3 isconnected through capacitor 4| to cell anode 36. Voltage to plate 39 oftube V3 is supplied from battery 5 through the primary of transformerT3, the secondary of which serves to deliver the output of tube V3 to asubsequent circuit, in the samemanner as the secondaries-of transformersTI and T2.

Plate 39 of V3 is shunted to ground line G through capacitor 43 inseries with resistor 44 and potentiometer 45. The circuit connections oftubes VI and V2 are identically as in Fig. 3, except that the grid ofeach of these tubes is provided with an additional grid leak resistor,grid II with resistor 52 brought to tap 53 of potentiometer 45, grid l2with resistor 50 brought to tap 5| of potentiometer 45. Likewise grid 38of tube V3 is provided with two grid leak resistors, Resistor 46 isbrought to tap 41 of potentiometer 35, resistor 48 to tap 49 ofpotentiometer 32.

Thus in Fig. 4, the gridof each amplifying-tube is enabled to obtain acompensating voltage of proper phase and magnitude from the platecircuits of the other tubes. Each grid separately would, without thecompensation provided :by this invention, be afiected by the desired.and undesired voltage components, the .latteraris'ins from crosstalk incell l and being of much lower value than the desired component.Potentiometers 32, 35 and 45 are doubly tappedto supply compensatingvoltages: potentiometer 32 supplies such voltages to grids I2 and 38;potentiometer '35, to grids II and 38; potentiometer 45, to grids I land [2. While the separate potentiometer settings are not whollyindependent, appropriate settings of the taps on each of the threepotentiometers can readily be found, since each grid voltage has adesired component greatly predominant over the undesired components.

The resistances of potentiometers 32, 35 and 45 may be determined by therule already given in the description of Fig. 3. The two grid leakresistors shown in Fig. 4 paralleled on the grids of each tube are,however, each 1 megohm, so that they combine to make the impedance ofthe grid-to-ground connection of each tube 0.5 megohm as in Figs. 1 and3. v 7

While the invention has been described with reference to photoelectriccells having in the same glass envelope plural anodes and a cathodecommon thereto, it will be understood that the method of the inventionis not limited to the devices illustrated in the foregoing description.The method of the invention may be applied in the case of multipleelectrodes of one polarity, whether positive or negative, with a commonelectrode of opposite polarity or with physically separate electrodes ofopposite polarity connected to a com mon terminal. The electrodes inquestion may be those of any type of photosensitive device, and may ormay not be housed in a common envelope.

What is claimed is:

1. In a light sensitive circuit including a photoelectric cell havingdual anodes and a cathode common to said anodes, a source of lightsignals associated with the first of said anodes, a second source oflight signals associated with the second of said anodes, an amplifyingtube individual to each of said anodes, a common ground connection tothe cathode circuits of said amplifying tubes, conductive couplingbetween said common ground connection and the common cathode of saidphotoelectric cell, couplin between the grid of the first of saidamplifying tubes and the first anode of said photoelectric cell, andcoupling between the grid of the second of said amplifying tubes and thesecond anode of said photoelectric cell, means for electricallyreproducing said light signals independently of each other, including agrounded potentiometer traversed by currents corresponding to the secondof said light signals and a resistive coupling between the grid of thefirst of said amplifying tubes and a selected tap on said potentiometer.

2. In a light sensitive circuit including a photosensitive device havingdual electrodes of one polarity and an electrode of opposite polaritycommon to said dual electrodes, a source of light signals associatedwith the first of said dual electrodes, a second source of light signalsassociated with the second of said dual electrodes, an amplifying tubeindividual to each of said dual electrodes, a common ground connectionto the cathode circuits of said amplifying tubes, conduct ve couplingbetween said common ground connection and the common electrode of saidphotosensitive device, coupling between the grid of the first of saidamplifying tubes and the first of said dual electrodes, and couplingbetween the grid of the second of said amplifying tubes and the secondof said dual electrodes, meansfor electrically reproducing said lightsignals independently'off each other,- including a groundedpotentiometer traversed by currents corresponding to the second of saidlight signals and a resistive coupling between the grid of the first ofsaidamplifying tubes and a selected tap on said potentiometer.

3. In a light sensitive circuit including a photoelectric cell havingdual anodes and a cathode common to said anodes, a source of lightsignals individually associated with each of said anodes, means forelectrically reproducing said light signals independently of each other,including an amplifying tube individual to each of said anodes, couplingbetween the grid of each of said tubes and the corresponding one of saidanodes, a common ground connection to the cathode circuits of saidtubes, a potentiometer connected between said common ground connectionand the common cathode of said photoelectric cell, and a resistivecoupling between the grid of each of said tubes and a selected tap onsaid otentiometer.

4. In a light sensitive circuit including a photoelectric cell havingdual anodes and a cathode common to said anodes, a source of lightsignals individually associated with each of said anodes, means as inclaim 3 for electrically reproducing said light signals independently ofeach other, wherein the potentiometerconnected'between the common groundconnection and the common cathode of said photoelectric cell has aresistance of the order of 10 per cent of that of the resistive couplingbetween the grid of either amplifying tube and the corresponding tap onsaidpotentiometer. c r

5. In a light sensitive circuit including a photoelectric cell havingdual electrodes of one polarity and a common electrode of oppositepolarity, a source of light signals individually associated with each ofsaid dual electrodes,'means for electrically reproducing said lightsignals independently of each other, including an amplifying tubeindividual to each of said duel electrodes, coupling between the grid ofeach of. said tubes and the corresponding one of said dual electrodes, acommon ground connection to the cathode circuits of said tubes, 2.potentiometer connected between said common ground connection and thecommon electrode of said photoelectric cell, and a resistive couplingbetween the rid of each of said tubes and a selected tap on saidpotentiometer.

6. In a light sensitive circuit including dual photosensitive deviceshaving electrically independent electrodes of one polarity andelectrically connected electrodes of opposite polarity, a source oflight signals individually associated with each of said electricallyindependent electrodes, means for electrically reproducing said lightsignals independently of each other, including an amplifyingtubeindividualto' each of said electrically independent electrodes, couplingbetween the grid of each of said tubes and the'corresponding one of saidelectrically independent electrodes, a common ground connection to thecathode circuits of said tubes, a potentiometer connected between saidcommon ground connection and the electrically connected electrodes ofsaid photosensitive devices, and a resistive coupling between the gridof each of said tubes and a selected tap on said potentiometer. 7. In alight sensitive circuit including aphotoelectric cell having dual anodesand a cathode common to said anodes, a source of light signalsindividually associated with each of said anodes, means for electricallyreproducing said light signals independently of each other, including anamplifying tube individual to each of said anodes, coupling between thegrid of each of said tubes and the corresponding one of said anodes, acommon ground connection to the cathode circuitsof said tubes and to thecommon cathode of said photoelectric cell, a shunt circuit comprisingcapacitance and resistance in series individually connecting the anodeof each of said amplifying tubes to said common ground connection, and aresistive coupling between the grid of each of said tubes and a selectedtap on the resistance included in the shunt circuit connecting the anodeof the other of said tubes to said common ground connection.

8. In a light sensitive circuit including a photoelectric cell havingdual anodes and a cathode commonto'said anodes, a source of lightsignals individually associated with each of said anodes, means as inclaim '7 for electrically reproducing said light signals independentlyofeach other, wherein each shunt circuit connecting the anode of anamplifying tube to the common ground connection includes a resistor ofwhich the resistance bears to that of the tube load a ratio of the orderof 100 to 1, in series with a condenser of which the capacity inmicrofarads is of the order of onetenth the resistancein megohms of saidresistor.

9. In a light sensitive circuit including a photoelectric cell havingdual anodes-and a cathode common to said anodes, a; source of lightsignals individually associated with eachof said anodes, means as inclaim 7 for electrically reproducing said light signals independently'ofeach other, wherein each shunt circuit connecting the anode of anamplifying tube to the common ground connection comprises in series aresistor of which the resistance bears to the resistance of the tubeload a ratio of the order of 100 to 1, a condenser of which the capacityin microfarads is of the order of one-tenth the resistance inmegohms ofsaid resistor, and a potentiometer of which the resistance is of theorder of per cent of that of said resistor.

10. In a light sensitive circuit including aphotoelectric cell havingplural anodes and a cathode common to said anodes, a source of lightsignals individually associated with each of said anodes, means forelectrically reproducing said light signals independently of each other,including an amplifying tube individual to each of said anodes, couplingbetween the grid of each of said amplifying tubes and the correspondingone of said anodes, a common ground connection to the cathode circuitsof said tubes and to the common cathode of said photoelectric cell, ashunt circuit comprising capacitance and resistance in seriesindividually connecting the anode of each of said tubes to said commonground connection, and resistive coupling between the grid of each ofsaid tubes and a selected tap on the resistance included in each shuntcircuit connecting the anode of another of said tubes to said commonground connection.

11. In 'a light sensitive circuit including a photo-sensitive devicehaving plural electrodes of one polarity and an electrode of oppositepolaritycommon to said plural electrodes, a source of light signalsindividually associated with each of said plural electrodes, means forelectrically reproducing said light signals independently of each other,including amplifying tube pling between the gridof each of saidamplifying tubes and the corresponding one of said plural electrodes, acommon ground connection to the" cathode circuits of said" tubes and tothe common electrode of said photosensitive device, a shunt circuitcomprising capacitance and resistance in series individually connectingthe anode of each ofsaid tubes to said common ground connection, andresistive coupling between the grid of eachofsaid tubes and a selectedtap on the resistance including in'e'ach shunt circuit connecting theanode of another of said tubes to saidcommon ground connection.

12. In a light sensitive circuit including a plurality ofphotosensitive-devices having electrically independent electrodes of onepolarity and electrically connected electrodes of opposite polarity, asource of light signals individually associated with each of saidelectrically independent electrodes, means for electrically reproducingsaid light signals independently of each other, including an'amplifyingtube individual to each of said electrically independent electrodes,coupling be-' tween the grid ofeach of said tubes and the correspondingone of said electrically independent electrodes, at common groundconnection to the cathode circuits of said tubes and to the electricallyconnected electrodes of said photosensitive devices, a shunt circuitcomprising capacitance and resistance in series individually connectingthe anode of each of said amplifying tubes to said common groundconnection, and resistive coupling between the grid of each of saidtubes and a selected tap on the resistance included in each shuntcircuit connecting the anode of another of saidtubes to said commonground connection.

13. In a light sensitive circuit including a photoelectric cell havingdualanodes and a cathode common to said anodes, together with anamplifying tube individual to each of said anodes, the the method ofcompensating crosstalk between the two anode-cathode paths of saidphotoelectric cell which comprises introducing into the grid circuit ofeach of said amplifying tubes a controlled voltage derived from thecurrents generated in said photoelectric cell opposite in phase butcorresponding in magnitude to the crosstalk voltage normally present insaid grid circuit.

14. In a light sensitive circuit including a photoelectric cell havingplural anodes and a cathode common tosaid anodes, together with anamplifying tube individual to each of said anodes, the method ofcompensating crosstalk among the several anode-cathode paths of saidphotoelectric cell which comprises introducing into the grid circuit ofeach of said amplifying tubes a controlled voltage derived from thecurrents generatedin said photoelectric cell individually opposite inphase but corresponding in magnitude to each of the crosstalk voltagenormally present in said grid circuit.

15. In a light sensitive circuit including aphotosensitive device havingplural electrodes of one polarity and an electrode of opposite polaritycommon to said plural electrodes, together with an amplifying tubeindividual to each of said plural electrodes, the method of compensatingcrosstalk among the several anode-cathode paths ofcsaidphotosensitivedevice which comprises introducing into the grid circuit of each of saidamplifying tubes a, controlled voltage derived from thecurrentsgenerated.in said photosensitive device individually opposite in phase butcorresponding in magnitude to each of the crosstalk voltages normallypresent in said grid circuit.

16. In a light sensitive circuit including a photosensitive devicehaving plural electrodes of one polarity and an electrode of oppositepolarity common to said plural electrodes, the method of compensatingcrosstalk among the several anodecathode paths of said photosensitivedevice which consists in neutralizing each crosstalk voltage derivedfrom the currents generated in said light sensitive circuit by a voltagecomprising plural components at least one of which is of correspondingmagnitude but opposite phase to said crosstalk voltage.

WALTER J. ALBERSHEIM.

